This might be the game's way of telling me to get the fuck on with Crowbar, but it's not the boss of me. It appears that when I load my working copy of PL Scarlatine, Hammer will crash after the loading bar has reached 100%. Interestingly, if I load the map in the HL2 version of Hammer it gives me the message "Model models/props_mining/track_arc_90degree_256.mdl not found and models/error.mdl couldn't be loaded" before crashing out. Deleting these from the VMF manually doesn't help for the TF2 version. I'll keep investigating.